Point Groups

Point groups are a method of classifying the shapes of molecules according to their symmetry elements.

Before we can talk about point groups, we need to describe the basic elements of symmetry. These are the proper axis of symmetry (or just axis of symmetry), improper axis of symmetry, plane of symmetry, and inversion center (or point of symmetry).

The main classes of point groups are C, D, S, T, O, and I. The first two classes are most common. Each of these classes is subdivided into different point groups.

You should also remember that most compounds are conformationally mobile (i.e., constantly changing their shape), and, as a result, the point group of a compound can depend on the time scale. For example, at a very short time scale, 1-propyne (HC≡CCH3) is in the C3v point group, but at longer time scales, rapid rotation about the C–CH3 bond puts it in the C∞v point group. Cyclohexane is in the D3d point group at short time scales, when it is in a single chair form, but at longer time scales, at which it is in rapid equilibrium between its two chair forms, it is in the D6h point group.

Additional vocabulary help

You might want to look at some definitions of stereoisomers.

Or you might want to look at the stereochemical glossary.

Or you might want to look at some examples of the different kinds of stereoisomers.

Or you might want to look at a flow chart showing how to determine the isomeric relationship between two structures.